When he leaves, one of my assistants shinnies up the tree to take some fruit for us to weigh and dry out. I pick up some loose durians that Roman and Rob have knocked down. Later in the laboratory I'll be able to figure out how many calories they have consumed today.

I spread plastic sheets beneath animals sleeping in their tree nests to collect urine to analyze for hormones and signs of disease. In bringing such scientific techniques into the forest, my goal is to gain a deeper understanding of orangutans without intruding on their life in the wild.

When Times Are Lean

Doing a little jungle gymnastics, young Misha hangs by one arm, peeling ribbons of bark from the tree as her mother, the upside-down Marissa, does the same.

After several months of superhigh fruit production the forest now offers slim pickings, and orangutans scramble to find enough to eat. Males sometimes journey to the forest floor (though females rarely do); Jari Manis descends to the ground to suck termites from their nest.

Though they now have to turn to low-quality fruit and vegetation such as bark, leaves, or the celery-like Pandanus that Beth has broken off, they make do.

Such periods of abundance and scarcity have helped shape orangutan evolution. As with humans, orangutans store fat when food is abundant. By measuring the by-products in their urine, I've found that during periods of scarcity they produce ketones—telling me that they are burning up their fat deposits. The extra calories they stored when fruit was plentiful are now helping them survive.

Conflict Between Males

When we came upon Rocky on the forest floor one day, we saw new gashes on his right shoulder. His wounds could have been made only by another male's canine teeth, like those displayed by Rob, in the infrequent but sometimes deadly fights between males. Several weeks later when I was following Rob, I came upon Rocky again. He had become severely emaciated and was in no shape for an encounter with Rob. The two grappled and rolled on the ground until Rob finally left, leaving Rocky even further exhausted.

The next morning we found Rocky curled in a ball on his side, his matted hair giving little cover to his protruding ribs. Around midday he managed to shuffle over to a small tree, where he grabbed a handful of fruit and collapsed again, slowly chewing without lifting his head from the ground. Sadly, I knew that he didn’t have long to live.